1. Field of the Invention
This invention relates to a fiber optic mode conditioner and, more particularly, to a method and apparatus for achieving an equilibrium light distribution in a multimode optical fiber.
2. Description of Related Art
In recent years, fiber optic links have been used to an increasing extent to replace existing data communication links using copper conductor cables. Among the advantages of fiber optic cables over copper cables are increased information capacity for the same cable size, electrical isolation of the source from the destination, the absence of electromagnetic or RF interference, and improved data security and integrity. Typically, such fiber optic links are used either within a computer system, such as between a host processor and a device control unit and possibly one or more intermediate switches, or in such intersystem uses as telecommunications.
Fiber optic cables are essentially waveguides in which a core is surrounded by a cladding having an index of refraction lower than that of the core. Light energy travelling along a fiber optic conductor thus exhibits propagation modes, in the same general manner as, for example, microwave radiation in metal waveguides. Generally, fiber optic conductors may be classified as either single-mode conductors, in which the energy is confined to a single mode of propagation, or multi-mode conductors in which a fundamental mode as well as higher-order modes are propagated.
The spatial and angular light energy distribution of a multimode fiber varies considerably depending on fiber core diameter, numerical aperture (NA), the length of the fiber and the source launch conditions. Beyond approximately 2 km the light distribution of a typical fiber reaches an equilibrium state known as the equilibrium mode distribution (EMD). This light distribution can be characterized as a 70/70 distribution, meaning 70% of the core is filled and 70% of the angles are present. Once the EMD is achieved the light distribution no longer depends upon the source launch conditions. If the length of the fiber is less than 2 km a significant departure from the EMD can exist, and the light distribution in the fiber will be affected by the source launch conditions.
The amount of loss in a fiber optic link is affected primarily by fiber attenuation and connector loss. Both connector loss and, to a lesser extent, the fiber attenuation are dependent on the light energy distribution in the fiber. Hence, if an EMD does not exist, the source launch conditions may have a strong influence on the link loss. Depending on the link configuration, measurement variability from different sources on the same link may be as high as 3-4 dB.
In general, prior-art methods of achieving an equilibrium mode distribution (EMD) on an optical fiber do so by starting with an optical signal containing a large order of modes (approximately 900 modes for a 62.5-micron fiber) and using suitable mode-filtering techniques to remove all but the strongly guided modes (approximately 225). Thus, the Electronic Industries Association (EIA) recommends a procedure (FOTP-50) which is supposed to remove the source dependence of these link measurements. The procedure requires the source jumper to be wrapped a number of turns (typically 3 to 5) around a mandrel which does not stress the cable (typically 16 to 25 mm in diameter). This method improves the results only slightly. Although wrapping more turns around a smaller mandrel diameter may improve the result by stripping more modes, the minimum-cable-bend-radius specification must be violated to make any appreciable difference. Using eight turns around a 6-mm diameter mandrel still leaves an unacceptable 1.7 dB of variability in lab measurements.
The EIA alternatively recommends using either an adjustable launch condition with an adjustable length of fiber (typically 500-1000 m) spliced onto the test fiber for mode equalization, or a sophisticated beam optics approach that requires the use of an optical bench with a variety of lenses, apertures, filters and special light sources. These techniques are not well suited for field measurements.
Still another technique involves the use of a special 200/250-micron fiber as a mode scrambler/stripper. Although this technique does not produce an EMD, it yields excellent results when used for removing source dependence. However, the fiber is not standard and has limited availability because of its nonstandard size.